1. Field of the Invention
This invention relates to an ophthalmological diagnosis apparatus, and more particularly to an ophthalmological diagnosis apparatus whereby the eye fundus is illuminated by a beam of laser light and the movement of a speckle pattern formed by diffused laser light reflected by the tissue of the fundus at an image plane which is conjugate with the eye fundus is detected as fluctuation in the speckle light intensity to measure the blood flow state for ophthalmological diagnosis.
2. Description of the Prior Art
Conventional methods that employ laser light to measure the state of the blood flow in the eye fundus include those disclosed in Japanese Pat. Laying-open Nos. 55(1980)-75668 and 56(1981)-49134. Both of these are methods for determining blood flow velocity based on the laser Doppler effect, so in each case it is therefore necessary to detect the frequency shift of the laser light caused by the Doppler effect. This can be done using either of two arrangements. One comprises splitting the incident laser beam into two beams forming equal angles with respect to the optical axis of the incident laser beam and directing the split beams into the eye to be examined so that they intersect precisely at the position of the eye fundus blood vessel concerned. The other arrangement is to detect laser light scattered by the eye fundus blood cells from two different directions. In both cases the optical system is complex and needs to be high-precision. In addition, the fact that the angle of beam incidence or light detection has to be known in advance, the fact that a laser beam adjusted to a beam diameter that is substantially equal to the diameter of the blood vessel concerned (which generally measures between several tens and several hundred micrometers) has to be directed onto the blood vessel with high precision, and the fact that the person undergoing examination has to be kept motionless during the period of measurement make these methods extremely difficult to apply clinically and impair the repeatability and reliability of the results thereby obtained.
In order to overcome the aforementioned drawbacks the present inventors have submitted patent applications (Appln. Nos. 61(1986)-38240 and 61(1986)-67339) for a method and apparatus that utilize the laser light speckle phenomenon. According to this method, a laser beam of a prescribed diameter that is larger than the diameter of the blood vessels is used to illuminate the eye fundus so that light scattered and reflected by blood cells in the eye fundus tissue forms a laser speckle pattern. With the plane of the eye fundus defined as the object plane, the movement of the speckle pattern formed at the Fraunhofer diffraction plane with respect to the object plane or at an image plane that is conjugate with respect to the eye fundus is then detected as fluctuations in light intensity by means of finite detecting apertures, and is analyzed to thereby measure the state of blood flow in the eye fundus.
However, with this method, the speckle pattern formed at the Fraunhofer diffraction plane consists of superposed fields of light scattered from all of the scattering points within the region of the fundus illuminated by the laser beam. As such, light scattered from blood cells in the target blood vessel is superposed with light scattered from the blood cells of adjacent blood vessels, making it difficult to evaluate the blood flow in any one specific blood vessel. In addition, light scattered by the walls of blood vessels and surrounding tissue is also included, which forms optical background noise with respect to the light that is scattered by the blood cells in the target blood vessel. This has made it difficult to detect signals having a good S/N (signal/noise) ratio at the Fraunhofer diffraction plane.